Any idea what trickled down the right side of Ceraunius Tholus and pooled at the bottom?
The only reference I can find for the phenomenon is this one at
http://www.solarviews.com/cap/mars/ceraun.htm :
A major impact crater, just above Ceranius Tholus, postdates the plains material and volcano. However, a prominent delta of probable volcanic material was emplaced within the impact crater at the mouth of a sinuous channel that extends up the flank of Cerauius Tholous to the summit crater.
The theory is that the volcano was dormant when an impact occurred by it, which "awoke" the volcano. It then erupted material which ran down into the impact crater.